Digital communications use a variety of carrier signal modulation schemes. Numerous of these utilize in-phase (I) and quadrature (Q) signals to modulate baseband information onto a radio frequency (RF) carrier. The respective I and Q signals are phase-orthogonal to one another and are readily represented in a Cartesian coordinate system. However, noise filtering and other performance considerations have motivated the development of other modulation schemes known as polar modulation. Therein, time-varying amplitude (A) and phase angle (ED) signals are used to modulate baseband information onto a RF carrier. Polar modulation generally achieves better signal quality and less electrical current consumption compared to IQ modulation techniques.
FIG. 1 graphically depicts an illustrative polar modulation scheme in accordance with known techniques for a four-symbol digital communication environment. Digital baseband information—that is, the digital intelligence to be modulated onto a carrier wave—is represented by a time-varying amplitude signal 100 and a time-varying phase signal 102.
FIG. 2 is a block diagram depicting an illustrative polar modulation system 200 in accordance with known techniques. The system 200 includes a phase modulator 202 configured to modulate the phase of a radio frequency (RF) carrier signal in accordance with a baseband phase signal input. The system 200 also includes a mixer 204 that receives the phase modulated RF carrier signal from the phase modulator 202. The system 200 further includes an amplitude modulator 206 configured to provide an amplitude modulation signal to the mixer 204 in accordance with a baseband amplitude signal input. The mixer 204 modulates the amplitude of the phase modulated RF carrier signal in accordance with the amplitude modulation signal from the amplitude modulator 206. The mixer 204 thus provides a polar modulated carrier signal.
Returning to FIG. 1, the illustrative polar modulated carrier signal is graphically depicted in a constellation diagram 104. The constellation diagram 104 includes four, two-bit digital symbols 106, 108, 110 and 112, respectively. In this way, the constellation diagram 104 can be referred to as a constellation of four symbols 106-112, each represented by a particular polar modulation of the RF carrier signal. Under the present illustration, a stream of digital baseband information is modulated onto an RF carrier signal one symbol—two digital bits—at a time. It is important to note that a polar modulation system (e.g., system 200) must be able to accommodate “zero crossings” of the digital baseband information during such an operation.
By way of example, and not limitation, the constellation diagram 104 depicts an operational instance wherein the digital information “1100” is modulated onto the RF carrier signal. Thus, the symbol 106 and then the symbol 110 must be sequentially modulated onto the RF carrier. In doing so, the baseband amplitude signal 100 swings from full value, to zero, and then back to full value in the time domain, an operation readily accommodated by the amplitude modulator (e.g., 206). However, the baseband phase signal 102 is required to instantaneously shift one-hundred eighty degrees in the time domain—a situation referred to herein as a “zero crossing” scenario. As a result, a compliant phase modulator (e.g., 202) must accommodate nearly infinite frequencies—something impossible to realize thus far without distortions due to the limited bandwidth inherent to known real-world implementations. While the illustrative polar modulation scenario described above is set in the context of four digital symbols, it is to be appreciated that other polar modulation schemes (and their corresponding constellations) having other symbol counts (e.g., eight, sixteen, etc.) are contemplated herein.
FIG. 3 graphically depicts phase modulator 202 output signal characteristics during zero crossing by way of respective signal plots 302 and 304. In any case, polar modulation methods and systems that address the foregoing considerations are desirable.